Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for increasing a low-frequency content of seismic waves generated during a seismic survey.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of the earth's subsurface, information that is especially helpful in the oil and gas industry. Reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A land configuration for performing a seismic survey is illustrated in FIG. 1, which shows a system 100 that includes plural receivers 102 positioned over an area 104 of a subsurface to be explored and in contact with, or below the surface 106 of the ground. A number of seismic sources 108 (e.g., vibrators) are also placed on the surface 106 in an area 110, in a vicinity of the area 104 of the receivers 102. Alternatively, sources 108 may be buried under surface 106. A recording device 112 is connected to the plurality of receivers 102 and placed, for example, in a towed trailer, recording truck or other vehicle 114. Each source 108 can be composed of a variable number of vibrators, typically between one and five, and can include a local controller 116. A central controller 118 can be provided to coordinate the shooting times of sources 108. A global positioning system (GPS) 120 can be used to time-correlate the firing of sources 108 and/or seismic data recorded by receivers 102.
With this configuration, sources 108 are controlled to generate seismic waves, and the plurality of receivers 102 records waves reflected by the oil and/or gas reservoirs and other structures.
However, existing vibratory source elements are not effective in the low-frequency range of the spectrum, mainly 1 to 10 Hz, and more particularly, in the low frequency range of 0.5 to 5 Hz. In other words, energy generated by present sources in the very low-frequency spectrum and the resulting reflected energy is too weak to provide the necessary signal-to-noise ratio required for its successful application in seismic imaging. The low-frequency energy range is useful in seismic exploration because it provides better depth penetration of the seismic signal, which is extremely valuable for imaging in complex geological settings, such as sub-salt, basalt or even dense carbonate. The success of advanced techniques such as seismic inversion, which is useful for data interpretation, requires energy in the low-frequency range.
Thus, there is increasing interest in extending the seismic survey's bandwidth to lower frequencies to facilitate later imaging processing steps like seismic inversion. In the recent past, low-dwell sweeps have been developed to boost the low-frequency output of seismic vibrators with some success; however, because of stroke limitations, the peak low-frequency amplitude levels are quite weak and the dwell times need to become very long. The challenge of delivering sufficient low-frequency energy and still producing enough energy at moderate and high frequency for imaging can prove quite difficult when the cost of a survey is also considered.
Thus, there is a need to obtain low-frequency range seismic data for seismic interpretation without affecting the moderate and high-frequency energy. To be able to record such data, the source needs to be adjusted/modified to generate such low-frequency content. Therefore, it is desirable to provide sources and methods capable of generating low-frequency energy.